Urethane polymers have found widespread use in coatings for fabrics, plastics, wood, metal, and the like, due to their advantageous properties such as their good chemical resistance, abrasion-resistance, toughness, elasticity and durability, and their ability to cure rapidly. Conventionally, polymeric urethane coatings have been applied as solutions in, for instance, polar or aromatic hydrocarbon solvents. When the urethanes are of certain types, they may be compatible with aliphatic hydrocarbon solvents. While the coating is being dried, or cured, these solvents vaporize into the atmosphere.
Oil-modified, organic solvent-based, urethane compositions have been traditionally used as clear or pigmented air-drying, varnish-type coatings. These compositions are particularly used where higher levels of hardness and abrasion resistance and quick-drying capabilities are needed such as on floors or other objects subjected to wear. These properties are characteristics of the urethane components of the polymer while the air-drying capability is supplied by the oil portion, and these factors are generally balanced to give optimum properties. Sometimes alkyd resins, such as those prepared from phthalic anhydride oils and polyols, are physically or chemically incorporated in the compositions to lower cost or modify the products to develop particular performance properties. The resins may contain aliphatic hydrocarbon solvents to reduce viscosity and driers to accelerate autooxidation. When the coatings dry the solvent vaporized is an economic loss and, quite importantly, the vaporous solvent may pollute the atmosphere.
Aqueous, polyurethane coating compositions are, therefore, particularly desirable due to the low cost and availability of water. Moreover, aqueous coating compositions are advantageous since the evaporation of water into the atmosphere has little, if any, adverse effect on the environment whereas conventionally employed organic solvents may be toxic, flammable, odoriferous, or photochemically-sensitive, and thus, may be smog-formers in the daylight atmosphere due to photochemical oxidation. Furthermore, water which is readily available can be used to thin the water-based coating compositions and can be used in clean-up operations. However, urethanes generally are not compatible with water unless special ingredients and/or particular steps of manufacture are employed in their synthesis.
One approach to provide water-dispersible, polyurethane-containing compositions has been through the use of emulsifiers. This procedure generally suffers from the disadvantages that the dispersions are relatively unstable and the resultant films are water-sensitive.
Another way to prepare water-dilutable urethane coatings and thereby reduce or eliminate the organic solvent content is to make ammonium-neutralized, polyelectrolyte salts of the polymers. The presence of alcoholic cosolvents may be helpful in making the aqueous dispersion. Thus, when the ammonia or amine of the ammonium salt evaporates on drying, the salt deionizes and becomes water-insoluble. The film left behind from the dispersion then auto-oxidizes to a tough, water-resistant coating. In order to form the polyelectrolyte salts, pendant carboxylic groups may be introduced along the polymer chains of the oil-modified urethane polymer.
Introducing carboxylic groups into urethane polymers may be a difficult procedure due to the reactivity of these groups with the diisocyanate monomers or isocyanate-terminated prepolymers used to build the polymer chains. This reactivity can lead to reductions in the carboxylic group content of the product and formation of undesirable side products. Thus, the normal sources of carboxylic groups used in the preparation of alkyd resins can often not be effectively used to make these polymers. One must then inject the carboxylic group source after the isocyanate has reacted with the hydroxyl groups to form the urethane, or choose a special source of carboxylic groups that are unreactive with isocyanates at the temperatures used to form the polymers. Introducing the carboxylic group source after the reaction of the isocyanate groups is generally not practical due to the heat sensitivity and resulting discoloration of the urethane polymer. Even the lower temperatures, e.g., 250.degree. F.-300.degree. F., at which half-ester formation proceeds from acid anhydrides to leave carboxylic groups can be detrimental.
Proposals for incorporating carboxylic groups into urethane polymer structures in order for these groups to be neutralized to form suitable salt groups and thereby impart a degree of water-dispersibility to the urethane polymer are evident in U.S. Pat. Nos. 3,412,054; 3,479,310; and 3,870,684. Carboxylic groups in urethane polymers can result, however, in increased moisture sensitivity in the coatings or films formed from dispersions of such polymers. Aqueous polyurethane dispersions such as those exemplified in U.S. Pat. No. 3,412,054 in which a drying oil component is present in the initially-formed polyurethane are notably deficient in drying rate.
The use of dimethylol propionic acid (DMPA) which has a tertiary carboxylic group having low reactivity with isocyanate groups is described in U.S. Pat. No. 3,412,054 to prepare water-dispersible urethanes. In the procedure of the patent the DMPA in the presence of other polyols, if desired, is first reacted with fatty acids to make the DMPA compatible for reaction with the diisocyanate. The DMPA ester is then reacted with the diisocyanate to form the polymer, but this procedure gives inferior products. Alternatively, the patent describes the preparation of a polymer of TDI and DMPA in solvent, but nothing further is done with the polymer. These procedures do not give a suitable polymer for use in varnish-type coatings. The poor results are apparently because mixtures of the reactants are incompatible when the organic polyol is an oil-polyol alcoholysis product, and will not react. The present invention overcomes these difficulties.
Efforts have been made to improve the properties of coatings made with aqueous dispersions of urethane polymers. One procedure has been to incorporate into the urethane polymer structure, fatty acid-containing moieties derived, for example, from drying or semi-drying oils, to increase the hydrophobic properties of the resulting urethane polymer films. U.S. Pat. Nos. 4,046,729; and 4,066,591, disclose polymer modifications of this type. To the extent that these patents are concerned with the use of unsaturated polyesters, the latter are united as part of an isocyanate-terminated prepolymer. The efforts in the prior art to make aqueous dispersions, of oil-modified polyurethanes have been successful to varying extents, and at least for the most part the resulting dispersions or coatings have significant shortcomings when compared with those made from organic solvent-based polymer compositions.